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Experimental Studies of Thermal Diffusivities concerning some Industrially Important Systems
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The main objective of this industrially important work was to gain an increasing understanding of the properties of some industrially important materials such as CMSX-4 nickel base super alloy, 90Ti.6Al.4V alloy, 25Cr:6Ni stainless steel, 0.7% carbon steel, AISI 304 stainless steel-alumina composites, mould powder used in continuous casting of steel as well as coke used in blast furnace with special reference to the thermal diffusivities. The measurements were carried out in a wide temperature range covering solid, liquid, glassy and crystalline states.

For CMSX-4 alloy, the thermal conductivities were calculated from the experimental thermal diffusivities. Both the diffusivities and conductivities were found to increase with increasing temperature. Microscopic analysis showed the presence of intermetallic phases γ´ such as Ni3Al below 1253 K. In this region, the mean free path of the electrons and phonons is likely to be limited by scattering against lattice defects. Between 1253 K and solidus temperature, these phases dissolved in the alloy adding to the impurities in the matrix, which, in turn, caused a decrease in the thermal diffusivity. This effect was confirmed by annealing the samples at 1573 K. The thermal diffusivities of the annealed samples measured at 1277, 1403 and 1531 K were found to be lower than the thermal diffusivities of non-annealed samples and the values did not show any noticeable change with time. It could be related to the attainment of equilibrium with the completion of the dissolution of γ´ phase during the annealing process. Liquid CMSX-4 does not show any change of thermal diffusivity with temperature. It may be attributed to the decrease of the mean free path being shorter than characteristic distance between two neighbouring atoms.

Same tendency could be observed in the case of 90Ti.6Al.4V alloy. Since the thermal diffusivity increases with increasing temperature below 1225 K and shows slight decrease or constancy at higher temperature. For 25Cr:6Ni stainless steel, the thermal diffusivity is nearly constant up to about 700 K. Beyond that, there is an increase with temperature both during heating as well as cooling cycle. On the other hand, the slope of the curve increases above 950 K, which can be due to the increase of bcc phase in the structure. 0.7% carbon steel shows a decrease in the thermal diffusivity at temperature below Curie point, where the structure contains bcc+ fcc phases. Above this point the thermal diffusivity increases, where the structure contains only fcc phase. The experimental thermal conductivity values of these alloys show good agreement with the calculated values using Mills model.

Thermal diffusivity measurements as a function of temperature of sintered AISI 304 stainless steel-alumina composites having various composition, viz, 0.001, 0.01, 0.1, 1, 2, 3, 5, 7, 8 and 10 wt% Al2O3 were carried out in the present work. The thermal diffusivity as well as the thermal conductivity were found to increase with temperature for all composite specimens. The thermal diffusivity/conductivity decreases with increasing weight fraction of alumina in the composites. The experimental results are in good agreement with simple rule of mixture, Eucken equation and developed Ohm´s law model at weight fraction of alumina below 5 wt%. Beyond this, the thermal diffusivity/ conductivity exhibits a high discrepancy probably due to the agglomeration of alumina particles during cold pressing and sintering.

On the other hand, thermal diffusivities of industrial mould flux having glassy and crystalline states decrease with increasing temperature at lower temperature and are constant at higher temperature except for one glassy sample. The thermal diffusivity is increased with increasing crystallisation degree of mould flux, which is expected from theoretical considerations.

Analogously, the thermal diffusivity measurements of mould flux do not show any significant change with temperature in liquid state. It is likely to be due to the silicate network being largely broken down.

In the case of coke, the sample taken from deeper level of the pilot blast furnace is found to have larger thermal diffusivity. This can be correlated to the average crystallite size along the structural c-axis, Lc, which is indicative of the higher degree of graphitisation. This was also confirmed by XRD measurements of the different coke samples. The degree of graphitisation was found to increase with increasing temperature. Further, XRD and heat capacity measurements of coke samples taken from different levels in the shaft of the pilot blast furnace show that the graphitisation of coke was instantaneous between 973 and 1473 K.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006.
Keyword [en]
laser flash, thermal diffusivity, heat conduction, phonon, electron contribution, crystallisation degree, graphitisation
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-3864ISBN: 91-7178-251-6 (print)OAI: oai:DiVA.org:kth-3864DiVA: diva2:9767
Public defence
2006-03-16, Sal F3, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2010-06-29Bibliographically approved
List of papers
1. Thermal Diffusivity measurement of CMSX-4 alloy by Laser flash method
Open this publication in new window or tab >>Thermal Diffusivity measurement of CMSX-4 alloy by Laser flash method
2007 (English)In: International journal of thermophysics, ISSN 0195-928X, E-ISSN 1572-9567, Vol. 28, no 1, 109-122 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, thermal diffusivity measurements have been carried out on industrial samples of CMSX-4 nickel-base superalloy using the laser-flash method with emphasis on studying the effect of temperature and microstructure on the thermal diffusivity. The measurements were performed in the temperature range from 298 to 1623 K covering both solid as well as liquid ranges. Below 1253 K, the thermal-diffusivity values were found to increase with increasing temperature. Microstructural investigations of quenched samples revealed that below 1253 K, an ordered phase, usually referred to as the -phase was present together with the disordered fcc phase, often referred to as the γ phase. Between 1253 K and the solidus temperature, the phase was found to dissolve in the matrix alloy causing an increase in the disordering of the alloy, and thereby a small decrease in the thermal-diffusivity values. The thermal-diffusivity values of samples pre-annealed at 1573 K exhibited constancy in the temperature range from 1277 to 1513 K, which is attributed to the attainment of thermodynamic equilibrium. These equilibrium values were found to be lower than the results for samples not subjected to annealing. The thermal-diffusivity values of the alloy in the liquid state were found to be independent of temperature.

Keyword
laser-flash method, phonon conduction, thermal conductivity, thermal diffusivity, gamma and gamma ' phases, thermophysical properties
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-5403 (URN)10.1007/s10765-007-0163-5 (DOI)000245495400009 ()2-s2.0-34147115069 (Scopus ID)
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2017-11-21Bibliographically approved
2. Thermal Diffusivity Measurements of some Industrially Important Alloys by a Laser Flash Method
Open this publication in new window or tab >>Thermal Diffusivity Measurements of some Industrially Important Alloys by a Laser Flash Method
2007 (English)In: International journal of materials reseach, ISSN 1862-5282, Vol. 98, no 6, 535-540 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, thermal diffusivity measurements of three industrially significant alloys, viz. 90Ti.6Al.4V, stainless steel with 25Cr and 6Ni as well as plain carbon steel with 0.7 % carbon have been carried out as a function of temperature. The aim of this work is to study the effect of temperature and microstructure on the thermal diffusivities of these alloys. For the 90Ti.6Al.4V alloy, thermal diffusivity increases with temperature below 1225 K. Above this temperature, the values started decreasing probably due to the dissolution of Ti3Al intermediate phase in the matrix, which would result in an increase in the disorder of the structure. For 25Cr: 6Ni stainless steel, the thermal diffusivity is nearly constant up to about 700 K. Above this, there is an increase in the thermal diffusivities with temperature during the heating cycle, which was reproducible during thermal cycling. On the other hand, the slope of the curve increases above 950 K.

In the case of the 0.7 % carbon steel, the thermal diffusivity shows a decreasing trend with temperature below the Curie point for the alloy, where the alloy consists of bcc + fcc phases. Above this point, only the fcc phase is prevalent and the thermal diffusivity was found to increase with temperature. Heat transfer is carried out by lattice vibration (phonons) as well as electrons. The contribution of electrons varies depending upon the type of alloy. In this study, the highest electron contribution was found in 0.7 % carbon steel, while the lowest was in stainless steel. The thermal conductivity values of these alloys are in good agreement with the calculated values using the model proposed by Mills.

Keyword
thermal diffusivity; thermal conductivity; temperature dependence; phonon; electron contribution
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-5404 (URN)000247623300014 ()2-s2.0-34347256339 (Scopus ID)
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2012-02-14Bibliographically approved
3. Thermal Diffusivity of Sintered Stainless steel-Alumina Composite
Open this publication in new window or tab >>Thermal Diffusivity of Sintered Stainless steel-Alumina Composite
2006 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 37, no 4, 513-518 p.Article in journal (Refereed) Published
Abstract [en]

Thermal diffusivity measurements were carried out as a function of temperature on sintered AISI 304 stainless steel-alumina composites having various compositions (0.001, 0.01, 0, 1, 1, 2, 3, 5, 7, 8, and 10 wt pct Al2O3). The measurements were carried out between room temperature and 1473 K. The thermal diffusivity and the thermal conductivity were found to increase with temperature in all the composite specimens. The thermal diffusivity was found to decrease with increasing weight fraction of alumina. This tendency can clearly be seen at temperatures above 755 K. The experimental results are in good agreement with the simple rule of mixture, the Eucken equation, and the Ohm’s law model developed by Hayashi et al. at weight fractions of alumina below 5 wt pct. Beyond this composition, the thermal diffusivity/conductivity shows a large discrepancy from the models. This could probably be attributed to the accumulation of alumina particles during cooled pressing and sintering.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-5405 (URN)10.1007/s11663-006-0034-7 (DOI)
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2017-11-21Bibliographically approved
4. Effect of Crystallinity on the Thermal Diffusivity of Mould Fluxes for the Continuous Casting of Steels
Open this publication in new window or tab >>Effect of Crystallinity on the Thermal Diffusivity of Mould Fluxes for the Continuous Casting of Steels
2004 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 44, no 4, 691-697 p.Article in journal (Refereed) Published
Abstract [en]

The thermal diffusivities of mould flux having the glassy and crystalline states were measured as a function of temperature by the laser flash method to obtain the relationship between the thermal diffusivity and the degree of crystallization. The thermal diffusivities of the liquid mould flux were also measured to compare the data with those of the solid samples. The thermal diffusivity increases roughly linearly with an increase in the degree of crystallization. The thermal diffusivities of glassy and liquid samples having the ratios of NBO/T=1.48, 1.65 and 2.11 exhibit roughly the same values of 4.6*10-7 m2 s-1 and 4.0*10-7 m2 s-1, respectively. It is considered that because the silicate network is largely broken down, there is not a significant change of the structure for these samples.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-5406 (URN)
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2017-11-21Bibliographically approved
5. Studies on Graphitisation of Blast Furnace Coke by X-ray Diffraction Analysis and Thermal Diffusivity Measurements
Open this publication in new window or tab >>Studies on Graphitisation of Blast Furnace Coke by X-ray Diffraction Analysis and Thermal Diffusivity Measurements
2005 (English)In: Steel research, ISSN 0177-4832, Vol. 77, no 11, 763-769 p.Article in journal (Refereed) Published
Abstract [en]

High temperature XRD and thermal diffusivity experiments were employed to investigate the degree of graphitisation in blast furnace coke. The experiments were conducted between room temperature and 1473 K. X-ray diffraction studies were carried out in order to monitor the degree of graphitisation in the coke samples from the same campaign. In these cases, the degree of graphitisation was followed by the changes in the vertical dimension. The degree of graphitisation was found to be dependent on temperature. On the other hand, the graphitisation at each temperature was instantaneous, even at as low a temperature as 973 K and no dependency on time could be noticed. The thermal diffusivities of the coke samples taken from deeper level of the experimental blast furnace were measured using the laser-flash technique. The results from these measurements showed that thermal diffusivities increased with the degree of graphitisation, which in turn, can be affected by the level in the blast furnace at which the coke sample was taken. The present results indicate that the degree of graphitisation as followed by X-ray diffraction analysis as well as thermal diffusivities can be used to trace the thermal history of coke. DSC measurements show that the heat capacity of the coke increases with temperature towards the heat capacity of graphite, which could be due to the increasing the graphitisation degree of the coke.

Keyword
coke, graphitisation, heat capacity, laser flash, thermal diffusivity, XRD
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-5407 (URN)000242924200001 ()2-s2.0-33751396849 (Scopus ID)
Note

QC 20141126

Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2017-11-21Bibliographically approved

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